Absorbent webs which comprise entangled masses of fibers, i.e., fibrous webs, are well known in the art. Such webs can imbibe liquids, such as discharged body fluids, both by an absorption mechanism wherein fluid is taken up by the fiber material itself and by a wicking mechanism wherein fluid is acquired by, distributed through and stored in the capillary interstices between fibers. One means for improving the absorbency characteristics of such fibrous web structures is to incorporate therein superabsorbent material, such as as polymeric gelling material (also referred to as hydrogel-forming material superabsorbent polymers, etc.) which imbibe fluid. The superabsorbent material serves to retain fluid such as discharged body liquids. An absorbent structure of this type wherein hydrogel-forming materials in particulate form are incorporated into fibrous webs is disclosed in Weisman and Goldman; U.S. Pat. No. 4,610,678; Issued Sep. 9, 1986.
The improvement in absorbency provided by incorporation of absorbent gelling materials has permitted the realization of absorbent articles such as diapers which employ relatively thin absorbent cores and which are, therefore, relatively thin products. Thinner diapers are less bulky to wear and fit better under clothing. They are also more compact in the package, making the diapers easier for the consumer to carry and store. Compactness in packaging also results in reduced distribution costs for the manufacturer and distributor.
One such absorbent core configuration which is useful for use as the absorbent structure in relatively thin absorbent articles is disclosed in U.S. Pat. No. 4,765,780, issued Aug. 23, 1988 (Angstadt). This patent discloses absorbent articles, such as diapers, which have a two layer absorbent core configuration wherein the core comprises an upper primary layer and a lower dusting layer. The primary layer is an airlaid web of hydrophilic fiber material with a substantial amount of absorbent gelling material admixed therewith. The dusting layer comprises hydrophilic fiber material and, preferably, contains no absorbent gelling material.
Another absorbent core configuration is disclosed in Weisman/Houghton/Gellert, U.S. Pat. No. 4,673,402, issued Jun. 16, 1987. This patent discloses absorbent articles having a dual layer absorbent core. In the dual layer configuration, the core comprises an upper primary layer which is an airlaid web of hydrophilic fiber material, optionally with a small amount of polymeric gelling agent particles admixed therewith. The core also comprises an underlying insert layer which is an airlaid mixture of hydrophilic fiber material and a substantial amount of polymeric gelling agent particles. This insert layer is generally positioned toward the front of the absorbent article such that more than half of the polymeric gelling agent material in the article is found in the front half thereof. Absorbent articles having the particular dual layer configuration of the '402 patent can be prepared in the form of especially thin, highly effective, low leakage diaper products.
Notwithstanding the existence of absorbent cores as described above, there remains a need to provide absorbent cores with improved effective absorbent capacity. One way to theoretically do this would be to increase the level of polymeric gelling material in the absorbent core. Unfortunately, high levels of polymeric gelling material, (especially levels in excess of about 15%) in fibrous webs typically used in absorbent cores tends to induce a phenomena referred to as gel-blocking. Gel-blocking occurs when the polymeric gelling material located in regions first contacted with fluid increase in volume as a consequence of imbibing the fluid and forming the hydrogel. When polymeric gelling material concentration is too high, the hydrogel can block additional fluid from reaching other regions of the core having unused absorbent capacity. The occurrence of gel blocking can lead to leakage during usage of the absorbent article.
Polymeric gelling materials have been developed which can exhibit a reduced tendency to result in gel blocking. Such materials are described in RE U.S. Pat. No. 32,649, Apr. 19, 1988, Brandt/Goldman/Inglin. However, these improved polymeric gelling materials, and other superabsorbent materials, are subject to performance limitations of the web of cellulosic fibers in which particles of gelling material are distributed. In particular, upon initial wetting, the cellulosic fibers become highly flexible and the web tends to collapse to a higher density and, consequently, exhibits smaller average pore size. Whereas, pore size becomes smaller than the pore size in regions of the web not yet wetted, a capillary gradient is created which opposes efficient transport of fluids to the dry areas of the absorbent article.
Another reason why many absorbent articles such as diapers are subject to leakage is inability to absorb second and subsequent discharges of fluid even if the first fluid discharge has been effectively absorbed. Leakage due to second and subsequent discharges is especially prevalent during the night, when users commonly experience multiple discharges before being attended to. One reason for the inability of many absorbent articles to adequately handle multiple discharges of fluid, in addition to the reasons discussed above, is the inability of the absorbent core to transport discharged fluid away from the region of discharge once the absorbent capacity of that region has been reached. After a discharge of fluid occurs, the fluid tends to remain situated in the region proximate to the discharge. The occurrence of successive voiding of fluid creates a driving force to laterally transport the previous fluid and newly discharged fluid. However, actual performance of the absorbent article is limited by the ability to have the fluid transported to the farther reaches of the core. In this regard, even in the absence of polymeric gelling material, the overall absorbent capacity of conventional absorbent diaper cores is generally incompletely utilized prior to failure, i.e., leakage, of the absorbent article.
Yet another reason for leakage in conventional absorbent articles is the propensity of the cellulosic fibers conventionally utilized for fluid acquisition and distribution to collapse upon wetting, thus impairing permeability of the structures.
It is an object of this invention to provide superabsorbent-containing absorbent structures which can circumvent the problems of gel blocking and wet collapse and which can utilize an increased proportion of their absorbent capacity.
It is a further object of this invention to provide superabsorbent-containing absorbent structures which can acquire fluid rapidly in the region of discharge and transport the fluid over relatively large proportion of the absorbent structure storage area and, additionally, be capable of effectively acquiring and distributing discharged bodily fluid from second or other successive voiding.
It is yet another object of this invention to provide absorbent structures which are capable of meeting the objects described above which are of a relatively thin design.
One absorbent structure which has been suggested is described in U.S. Pat. No. 4,935,022, issued Jun. 19, 1990 to Glen R. Lash and Leonard R. Thompson. This patent discloses disposable absorbent articles comprising a layered absorbent core positioned between a backsheet and a topsheet, wherein the absorbent core comprises an upper layer of stiffened, twisted, curled cellulose fibers and requires from about 3% to 15%, by weight, of large particle absorbent gelling material and a lower layer of stiffened, twisted, curled cellulose fibers and from about 15% to 60%, by weight, of absorbent gelling material. The upper layer serves the principal purpose of acquisition and distribution of bodily fluid discharges. The stiffened, twisted, curled fibers are highly beneficial in this regard. The lower layer, which is necessarily smaller than the upper layer, is principally for fluid storage.
Another absorbent structure which has been proposed is described in U.S. Pat. No. 4,798,603, S. C. Meyer et al., issued Jan. 17, 1989, titled "Absorbent Article Having a Hydrophobic Transport Layer." As suggested by the title, this patent describes an absorbent article with a hydrophobic transport layer, made from known hydrophobic synthetic fibers. The transport layer is positioned between a topsheet and an absorbent body. The absorbent body is necessarily more hydrophilic than the transport layer. The purpose of the transport layer is to act as an insulating layer between the topsheet and the absorbent body, to reduce skin wetness. Regardless of whether the structures described therein meet this objective, the hydrophobic nature of the transport layer of U.S. Pat. No. 4,798,603 would be expected to have limited fluid acquisition and fluid transport properties due, at least in part, to the hydrophobicity of the layer. This would be particularly so for second and successive fluid discharges after which any optional surfactants have been washed away.
Notwithstanding the existence of absorbent articles of the type described above, there is a need to identify further improved configurations for absorbent articles which provide improved fluid distribution and acquisition performance, especially with respect to successive fluid discharges.
Accordingly, the present invention provides improved absorbent structures, and elements for use therein, as well as absorbent articles utilizing such structures, utilizing a multiple layer absorbent core that effectively and efficiently acquires the wearer's discharged body fluids upon initial and successive discharges, transports acquired fluid, from both initial and successive discharges over a relatively large proportion of the absorbent structure surface area, and stores such discharged fluids.